help

iCloud: Activation Lock - http://support.apple.com/kb/HT201365 - This is a security feature to deter re-selling of stolen devices.

Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it.

Get them to remove it or return the device for a refund.

If you can't change the Apple ID due to it being a second hand device and can't contact the owner, then you are out of luck.

Turn off Find My iPhone Activation Lock - Apple Support